The present invention relates to a TFT (thin film transistor) array substrate for use in a liquid crystal display apparatus and a liquid crystal display apparatus employing the same.
A liquid crystal display apparatus is generally arranged in that a display material such as liquid crystal is pinched between two substrates one of which is a TFT array substrate having formed thereon TFTs and the other is a counter substrate having formed thereon a color filter, black matrix and counter electrodes, wherein voltage is applied to the display material in a selective manner.
Such a TFT array is generally formed of inverted staggered type thin film transistors and is obtained by forming a gate wiring and gate electrodes on a glass substrate, forming a gate insulating film and semiconductor layer, and forming source electrode lines, source electrodes as well as drain electrode and pixel electrodes. Further, simultaneous with forming the gate electrode lines, electrically floating electrodes are formed at peripheral portions of the pixel electrodes using the same material as that for the gate electrodes for improving the aperture ratio of the pixels. The floating electrodes act to prevent leakage light from pixel periphery portions, and by the provision of these in a concurrent manner with the black matrix on the color filter (CF) side, unnecessary leakage light may be prevented at peripheral portions of the pixel electrodes so as to improve the display quality. With such an arrangement of forming a light shielding layer on the TFT array substrate side, the overlay accuracy between layers of a TFT array substrate can be made to be superior to the overlay accuracy between the CF substrate and the TFT substrate, so that the area of the light shielded portion can be decreased and a high aperture ratio of the pixels can be achieved.
However, since the metallic light shielding layer is provided to be floating in such an arrangement as shown in FIG. 10, coupling capacity C1, C4 between floating electrodes and source electrode lines and coupling capacity C2, C3 between floating electrodes and pixel electrodes exist in a serial manner with respect to each other (C1 and C2 in siries and C3 and C4 in series) so that these capacities caused display deficiencies such as cross talk or shot blurs.
FIG. 7(a) is a plan view of pixels of a conventional TFT array substrate, FIG. 7(b) a partially enlarged view of FIG. 7(a), FIG. 8 a sectional view of a TFT portion, and FIG. 9 a sectional view of a source electrode line portion.
A method for forming a TFT array substrate according to the prior art will now be explained. First, gate electrode lines 3 and floating electrodes 11 are formed of a metal material such as Cr. Then, there are respectively formed a gate insulating film 4, a non-doped amorphous silicon layer 5, and a contact layer 6 of phosphorus doped amorphous silicon. Amorphous silicon is patterned in a form of an island to form source electrode lines 7 and drain electrode 8. After removing unnecessary portion of the phosphorus doped amorphous silicon layer on a channel portion and forming pixel electrodes 9 of transparent electrodes, a passivation film 10 is formed to complete the TFT array.
In a conventional type liquid crystal display apparatus, since the black matrix is formed by arranging a light shielding film of floating electrodes in peripheral portions of the pixel electrodes to achieve a high aperture ratio, the coupling capacities between the source electrodes and floating electrodes as well as pixel electrodes become large so that a drawback was presented that display deficiencies such as cross talks, shot blurs or irregularities in luminance were caused.
The present invention has been made for the purpose of solving such conventionally known problems, and it is an object thereof to realize a TFT array for obtaining a liquid crystal display apparatus of large screen size, high precision and high aperture ratio free of display deficiencies such as luminance blurs or cross talks.
The TFT array according to the present invention is a TFT array substrate comprising:
a transparent insulating substrate;
a plurality of parallel gate electrode lines;
a plurality of parallel source electrode lines crossing to the gate electrode lines;
a plurality of TFTs located on each crossing point of the gate electrode lines and the source electrode lines;
a passivation film formed on the TFTs;
a plurality of transparent pixel electrodes formed on said passivation film corresponding to each said TFT and connected to a drain electrode of each said TFT via contact hole;
a plurality of floating electrodes which serve as light shielding layer made of a same layer as that of said gate electrode lines formed in peripheral portion of each said pixel electrode.
It is further characterized in that the overlay capacity between the pixel electrodes and floating electrodes is further decreased by forming the passivation film of organic film presenting low dielectric constant.
The thin film transistor array is further characterized in that light shielding characteristics are further increased and high aperture ratio is realized by the provision of extending the floating electrodes under the source electrodes, and in that differences between shots of capacities between source electrode lines and floating electrodes are eliminated by connecting portions of the floating light shielding film being arranged laterally with the source electrode lines between.
Employing the TFT array substrate according to the present invention, it is possible to arrange a liquid crystal display apparatus of high aperture ratio, of large screen size and of high precision free of occurrence of display deficiencies such as cross talks or shot blurs owing to coupling capacities of floating electrodes.